The British advertised for men willing to "face a rough and dangerous task", helping to boost the ranks of the Royal Irish Constabulary in policing the increasingly hostile Irish population. Black and Tans was the nickname given to the special auxiliary force used to fight Sinn Féin; the name came from a well-known pack of hounds in Limerick. They wore uniforms of khaki with black hats and belts, and were recruited in England. The first recruits arrived in Ireland on March 25, 1920, after three months of training. The government also raised another unit, the Auxiliary Division of the Constabulary, known as the "Auxiliaries[?]". The Black and Tans acted with the Auxiliaries in the government's attempts to break the IRA.

They were paid ten shillings a day - enough to motivate unemployed war veterans, and a lack of RIC uniforms led to the khaki and black uniforms. Since they had no training as policemen, their main role was to strengthen the military might of police posts, and they were viewed by Republicans as an army of occupation.

On the first Bloody Sunday in November 1920, where the IRA killed 14 British undercover officers, the Black and Tans surrounded a football match in Dublin. Shooting broke out and 12 people were killed. Republicans also refer to atrocities carried out by the Black and Tans in Cork city, which was heavily burned, and Balbriggan[?]. The mayor of Cork died after a 78-day hunger strike in prison in Brixton, London. The evidence suggests that the Black and Tans had adopted a shoot-to-kill policy, and their tactics have been described as "state-supported terrorism". There is no doubt as to the ferocity of the fighting and atrocities on either side, and feelings continue to run high regarding their actions.

In 1920 the Government of Ireland Act established the idea of two parliaments in Dublin and Belfast, subordinate to the London parliament. In 1921 the newly-elected parliaments sat for the first time. The Dublin parliament was dominated by Sinn Féin, and the Belfast parliament of Northern Ireland was opened by George V. An uneasy truce between the two sides came into effect.